Iran's supreme Leader gave a stern warning to the pro-reform opposition yesterday accusing it of violating the law by insulting the memory of the Islamic Republic's founder, Ayatollah Khomeini.

Declaring that opposition rallies were illegal, Ayatollah Ali Khamenei made clear he would not tolerate any more protests by reformers seeking to revive their challenge to President Mahmoud Ahmadinejad six months after his disputed re-election. Iran's top authority effectively sided with hardliners calling for tougher action against the opposition, which has continued to show defiance over a poll it says was rigged in the conservative incumbent's favour.

Tension has increased in Iran since student backers of opposition leader Mir-Hossein Mousavi clashed with security forces last Monday in Tehran in the largest anti-government demonstration in months. State television broadcast footage of what it said were opposition supporters tearing up and trampling on a picture of late revolutionary leader Khomeini during the protests. The opposition had earlier accused the authorities of planning to use the reported "desecration" of Khomeini's picture as a pretext for action to uproot the reform movement.

"They are openly violating the law, they insulted Imam Khomeini," Ayatollah Khamenei said in a hard-hitting televised speech. The leader, who swiftly endorsed Ahmadinejad's poll win in the summer, said "some people violated the law, created riots and encouraged people to stand against the system", in a clear reference to senior opposition figures.

"They also prepared the ground for the enemies of the revolution ... to insult the system," he added. The Supreme Leader said opposition rallies were illegal and urged the authorities to identify those behind the insult to Khomeini, who led the 1979 revolution. "The election is over. It was legal and they could not demonstrate their claim [of voting fraud]," he said.